Circuit breakers typically provide protection against the very high currents produced by short circuits. This type of protection is provided in many circuit breakers by a magnetic trip unit, which trips the circuit breaker""s operating mechanism to open the circuit breaker""s main current-carrying contacts upon a short circuit condition.
Modern magnetic trip units include a magnet yoke (anvil) disposed about a current carrying strap, an armature (lever) pivotally disposed near the anvil, and a spring arranged to bias the armature away from the magnet yoke. Upon the occurrence of a short circuit condition, high currents pass through the strap. The increased current causes an increase in the magnetic field about the magnet yoke. The magnetic field acts to rapidly draw the armature towards the magnet yoke, against the bias of the spring. As the armature moves towards the yoke, the end of the armature contacts a trip lever, which is mechanically linked to the circuit breaker operating mechanism. Movement of the trip lever trips the operating mechanism, causing the main current-carrying contacts to open and stop the flow of electrical current to a protected circuit.
Magnetic trip units used within circuit breakers as described above must be compact and reliable. In addition, such magnetic trip units must be adjustable to vary the level of overcurrent at which the circuit breaker trips. This adjustment is often attained by varying the distance between the magnet yoke and the armature. However, the trip set point range offered by adjusting the distance between the magnet yoke and the armature is limited due to the finite space inside the circuit breaker housing. In order to provide overcurrent protection for a wide range of trip set points desired for motor protection, manufacturers typically offer a selection of circuit breakers having different trip set point rangesxe2x80x94one circuit breaker offering a lower spectrum range of trip set points and a second circuit breaker offering a higher spectrum range of trip set points. Often times, however, a customer will choose a circuit breaker having an improper trip set point range for a particular application. In addition, costs associated with manufacturing and inventory are increased having two different circuit breakers in order to offer a circuit breaker that offers motor protection over a wide trip set point range. Therefore, it is desired that magnetic trip units offer a broader spectrum of overcurrent ranges (e.g., for use in motor protection), so that a single circuit breaker can offer a broader trip set point range to reliably trip at different levels of overcurrent.
The above and other drawbacks and deficiencies are overcome or alleviated by a magnetic trip unit for actuating a latching mechanism to trip a circuit breaker upon an overcurrent condition, the magnetic trip unit includes: an electrically conductive strap; a flux return component in electromagnetic communication with the electrically conductive strap; a tube disposed within the flux return component; a stator disposed at a first end of the tube and connected to the flux return component, the stator having a stator surface at one end; and a plunger slidably extending from a second end of the tube, the plunger comprises a plunger surface at one end facing the stator surface, the plunger further includes another end adapted to operably interact with the latching mechanism, the plunger is biased to a predetermined gap position.